Method and device for monitoring moisture content of an immersed solid dielectric material.
The present invention relates to the field of sensing devices and is more particularly concerned with a method and a device for monitoring the moisture content of a solid dielectric material immersed in a dielectric fluid.
As it is well-known in the art of high voltage transformers, the moisture content of the paper insulation used to insulate the wires in the windings affects the insulation characteristic of the transformer, the higher the moisture content is the lower its insulating characteristic is. High moisture content reduces the electrical resistance of the insulation which promotes local heating, especially during emergency overloading of the transformer, and increases the occurrences of small discharges that tend to degrade even further the paper insulation. Therefore, high moisture content shortens the insulation life duration.
It is also well known that the moisture content generally increases over time, mainly due to paper aging, infiltration and inspection. This increase of moisture content in the paper insulation forces the transformer to have a decreasing overload capacity over time.
The moisture content level of the paper insulation can be evaluated and/or estimated through the measurement of the moisture content level of the oil in which the windings, including the paper insulation, is immersed in. It is relatively easy to measure the moisture content of the oil since a lot of such devices are available in the market. Once the oil moisture content is known it is somewhat tedious to go through the analysis process in order to estimate the paper moisture content in different locations of the transformer. Accordingly, in practice, no one ever does or sees the necessity to do such an analytical estimation.
U.S. Pat. No. 5,343,045 granted to Gupta on Aug. 30, 1994, discloses a method and device for measuring moisture content of an absorbent material such as paper insulation in transformers. The device directly measures the humidity/moisture level in the absorbent material using an optical technique. The technique requires an end of an optic fiber to be installed almost in contact with the absorbent material or paper insulation.
Since the device provides a local direct measurement, is preferably located at a critical area of the transformer. Such a critical area is not always easily accessible and may require a somewhat complex and expensive installation of the device on the transformer. Furthermore, the location of such a critical area is not really known for sure and changes with the transformer loading. In order to get a general overview of the paper moisture content in the different locations of the transformer, either a plurality of devices or a calculated estimation is required. Notwithstanding the fact that such a device may provide reliability problems over time, especially problems associated with the maintaining of a proper position of the optical probe in proximity to the absorbent material.
The drawbacks associated with the prior art devices is their inability to provide an operator/user with a constant monitoring of the moisture level of the paper insulation using relatively simple and well-known temperature and humidity sensors. Furthermore, the assessment of the paper moisture content for different locations in the transformer is never performed due to its complexity. Nevertheless, such an assessment could prove to be beneficial over time in order to significantly increase the lifetime of the transformers as well as their continuous use at a substantially higher maximum overload capacity compared to their usual derated overload capacity required to compensate for all of the unmonitored behaviors thereof, for safety reasons.
Accordingly, there is a real need for an improved method and device for monitoring the moisture level of an immersed solid dielectric material.
Furthermore, an oil dryer-filter connected to the transformer is generally used to extract water from the oil. When such a filter becomes saturated in water, its efficiency is considerably reduced, if not null, and could affect the overload capacity of the transformer until it is replaced or cleaned up. Although the oil filters are generally provided with a known water extraction capacity, they sometimes lose efficiency before reaching the latter, which is bad for the transformer.
Accordingly, there is a need for continuously monitoring the proper functioning of the oil dryer-filter during operation of the high power transformer.
It is therefore a general object of the present invention to provide an improved method and device for monitoring moisture content of a solid dielectric material immersed in a dielectric fluid.
An advantage of the present invention is that the method and/or device for monitoring moisture content of an immersed solid dielectric material provide a relatively accurate estimate.
A further advantage of the present invention is that the method and/or device for monitoring moisture content of an immersed solid dielectric material to provide an estimate for different locations inside the transformer.
Yet another advantage of the present invention is that the device for monitoring moisture content of an immersed solid dielectric material is relatively stable over time.
Still another advantage of the present invention is that the device for monitoring moisture content of an immersed solid dielectric material operates over very large temperature and pressure ranges.
Still a further advantage of the present invention is that the device for monitoring moisture content of an immersed solid dielectric material can be easily installed or retrofitted on existing transformers, at accessible locations.
Another advantage of the present invention is that the method and/or device for monitoring moisture content of an immersed solid dielectric material are not affected by the presence of contaminants within the dielectric fluid.
A further advantage of the present invention is that the method and/or device for monitoring moisture content of an immersed solid dielectric material can be adapted by the user for use in different dielectric fluids and/or for different types of solid dielectric materials.
Still another advantage of the present invention is that the method and/or device for monitoring moisture content of an immersed solid dielectric material can provide all sorts of useful information concerning the status, or health, of the transformer(s).
Furthermore, the use of moisture detectors located upstream and downstream of the oil dryer-filter used to extract water from the oil allows for continuous monitoring of the proper functioning of the oil filter; the upstream moisture detector is preferably simultaneously used for the monitoring of the moisture content in the immersed solid dielectric.
Another advantage of the present invention is that the method and/or device for monitoring operation of a fluid dryer-filter calculate the rate of water extraction as well as the total amount of water extracted from the oil since the last filter cleaning or replacement.
According to an aspect of the present invention, there is provided a device for monitoring moisture content level of a solid dielectric material inside an enclosure, the solid dielectric material being immersed in a dielectric fluid, the dielectric fluid filling the enclosure, the solid dielectric material and the dielectric fluid having a respective moisture content, the solid dielectric material and the dielectric fluid having known water solubility properties varying with temperature thereof, the device comprises:
a moisture measuring means for measuring moisture content level of the dielectric fluid;
a temperature measuring means for measuring temperature level of the dielectric fluid; and
an electronic circuit means for computing the moisture content level of the solid dielectric material, the electronic circuit means being electrically connected to both the moisture measuring means and the temperature measuring means, the electronic circuit means having the known water solubility properties of the solid dielectric material and the dielectric fluid stored therein, the electronic circuit means processing the fluid moisture content level and the fluid temperature level so as to determine the solid dielectric material moisture content level.
Preferably, the electronic circuit means includes a displaying means for displaying the solid dielectric material moisture content level, the displaying means being electrically connected to the electronic circuit means.
Preferably, the device includes an operator interfacing means for an operator to interface with the electronic circuit means, the operator interfacing means being electrically connected to the electronic circuit means so as to allow the known water solubility properties of the solid dielectric material and the dielectric fluid to be provided to and stored in the electronic circuit means.
Typically, the electronic circuit means is remotely electrically connected to both the moisture measuring means and the temperature measuring means so as to allow the moisture measuring means and the temperature measuring means to be located in a generally inaccessible location.
Typically, the device includes an operator interfacing means for an operator to interface with the electronic circuit means, the operator interfacing means being electrically connected to the electronic circuit means, the electronic circuit means providing a sensor location menu through the operator interfacing means so as to allow an operator to select a specific location of both the moisture measuring means and the temperature measuring means within the enclosure, whereby the specific location affecting determination of the solid dielectric material moisture content level by the electronic circuit means.
Typically, the device includes:
a second moisture measuring means for measuring second moisture content level of the dielectric fluid, the electronic circuit means comparing the first and second dielectric fluid moisture content levels and calculating a relative difference therebetween relative to the first dielectric fluid moisture content level, the electronic circuit means displaying on the displaying means either a warning message when the relative difference is equal or larger than a predetermined value or the first solid dielectric material moisture content level when the relative difference is smaller than the predetermined value;
whereby the second moisture measuring means being a reference moisture measuring means to enable detection of malfunction of the device.
Preferably, the predetermined value is within the range of between fifteen (15) and twenty-five (25) percent, and most preferably is twenty (20) percent.
Preferably, the moisture measuring means is a capacitance-type moisture sensor.
Alternatively, the enclosure is in fluid communication with a dielectric fluid filter unit, the filter unit defining a filter unit inlet, a filter unit outlet and a filter therebetween, the moisture measuring means and the temperature measuring means being located at the filter unit inlet, the device further comprising a second moisture measuring means located at the filter unit outlet for measuring a second moisture content level of the dielectric fluid;
the electronic circuit comparing the first and second dielectric fluid moisture content levels and calculating a filter relative difference therebetween relative to the first dielectric fluid moisture content level, the electronic circuit means displaying on the displaying means either a filter warning message when the filter relative difference is equal or smaller than a filter predetermined value or the first solid dielectric material moisture content level when the filter relative difference is larger than the filter predetermined value;
whereby the second moisture measuring means being a filter saturation reference moisture measuring means to enable detection of saturation state of the filter of the filter unit.
According to another aspect of the present invention, there is provided a method for monitoring moisture content level of a solid dielectric material inside an enclosure, the solid dielectric material being immersed in a dielectric fluid, the dielectric fluid filling the enclosure, the solid dielectric material and the dielectric fluid having a respective moisture content, the solid dielectric material and the dielectric fluid having known water solubility properties varying with temperature thereof, the method comprises the following steps:
a) measuring moisture content level of the dielectric fluid using a moisture detector;
b) measuring temperature level of the dielectric fluid using a temperature detector; and
c) computing the solid dielectric material moisture content level using a processor electronic circuit, the electronic circuit being electrically connected to both the moisture detector and the temperature detector, the electronic circuit having the known water solubility properties of the solid dielectric material and the dielectric fluid stored therein, the electronic circuit processing the fluid moisture content level and the fluid temperature level so as to determine the solid dielectric material moisture content level.
Preferably, step a) includes measuring a second moisture content level of the dielectric fluid using a second moisture detector; step c) includes the electronic circuit comparing the first and second dielectric fluid moisture content levels and calculating a relative difference therebetween relative to the first dielectric, fluid moisture content level; and step d) includes displaying on the display either a warning message when the relative difference is equal or larger than a predetermined value or the first solid dielectric material moisture content level when the relative difference is smaller than the predetermined value; whereby the second moisture detector being a reference moisture detector to enable detection of malfunction of the device.